<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>

        var n1 = prompt('第一个数'); // 1
        var n2 = prompt('第二个数'); // 2

        var n = Number(n1) + Number(n2); // 3

        var result = '结果为: ' + n;

        alert(result);

    </script>
</body>
</html>